Spectrally narrowband, wavelength-tunable light sources are used, for example, in medicine, in particular in optical coherence tomography. Two-dimensional or three-dimensional images, for example, of human tissue, may be generated via optical coherence tomography. An important aspect of such images is the achievable resolution, i.e., inter alia, the number of pixels which result in an image. The quality of an image and its diagnostic usability are decisively influenced in this way, for example. In addition to a high detail reproduction, the period of time which is required for the generation of an image is also significant. In order to reduce the strain on a patient, for example, the period of time required for generating an image is to be reduced as much as possible. The combined requirements of high resolution and low duration mean that a high data rate is to be made possible. This places high requirements in particular on a light source usable for a tomographic method.
One possibility for implementing a narrowband, tunable light source comprises subsequently filtering a spectrally broadband radiation which is emitted by an incandescent lamp or arc lamp, for example. However, high radiation power is not achievable after the passage through the filter through this method. If the light source has a bandwidth of 100 nm and the filter has a bandwidth of 0.1 nm, for example, the loss due to the filter is approximately 99.9%, corresponding to an attenuation by a factor of 1000.
A tunable laser forms a further possibility for implementing a tunable light source. The laser has a laser medium, a resonator, and a tunable optical filter for this purpose. The laser medium can perform broadband spectral amplification. The tunable filter is situated in the resonator. Thus, only light which passes the optical filter and reaches the amplifier medium is amplified by the amplifier medium. Because a laser is based on amplification of the spontaneous emission, the radiation emitted by the amplifier medium is fed back in respect to the radiation reaching the amplifier medium. The tuning speed of the filter is a function, in addition to other factors, of the length of the resonator in particular. The greater the length of the resonator, the lower the achievable tuning speed. A reduction of the resonator length can cause increased intensity noise of the laser, however, and result in a greater frequency spacing of the modes of the laser. The maximum measuring range can be limited in applications of optical coherence tomography, abbreviated as OCT, in this way.
Another possibility for implementing a tunable light source comprises a Fourier domain mode-locked laser, abbreviated as FDML laser. Such a laser has an amplifier medium and at least one tunable filter in a resonator having a great length. The tuning speed of the filter is adapted to the length of the resonator for this purpose. In other words, the filter is transmitting again at a specific wavelength after a time which the light of this wavelength requires to pass through the resonator once. Because of this functional principle and the high speed of light, typical FDML lasers have resonator lengths in the range of multiple hundreds of meters up to several kilometers. Limitations of such an FDML laser in regard to compactness and accessible wavelength range result therefrom.
A method and an apparatus for optical coherence tomography using swept-frequency light sources are disclosed in the publication US 2008/0165366 A1.